Wind turbine blade provided with optical wind velocity measurement system

ABSTRACT

A wind turbine includes a number of blades and an optical measurement system comprising a light source, such as a laser, an optical transmitter part, an optical receiver part, and a signal processor. The light source is optically coupled to the optical transmitter part, which includes an emission point for emitting light in a probing direction. The optical receiver part comprises a receiving point and a detector. The optical receiver part is adapted for receiving a reflected part of light from a probing region along the probing direction and directing the reflected part of light to the detector to generate a signal used to determine a first velocity component of the inflow. The emission point is located in a first blade at a first radial distance from a center axis, and the receiving point is located in the first blade at a second radial distance from the center axis.

This is a National Phase Application filed under 35 U.S.C. 371 as anational stage of PCT/EP2010/068301 filed Nov. 26, 2010, and claimspriority benefit from European Application No. 09177500.7, filed Nov.30, 2009, the content of each of which is hereby incorporated byreference in its entirety.

TECHNICAL FIELD

The present invention relates to a wind turbine comprising a number ofblades including at least a first wind turbine blade extendingsubstantially radially from a hub on a main shaft having a substantiallyhorizontal centre axis, the blades together with the hub constituting arotor with a rotor plane, and which can be put into rotation by wind,and each blade having an innermost part comprising a root section of theblade and an outermost part comprising a tip section of the blade,wherein the wind turbine comprises an optical measurement systemcomprising a light source, such as a laser, an optical transmitter part,an optical receiver part, and a signal processor. The light source isoptically coupled to the optical transmitter part. The opticaltransmitter part comprises an emission point and is adapted for emittinglight in a probing direction from said emission point. The opticalreceiver part comprises a receiving point and a detector, wherein theoptical receiver part is adapted for receiving a reflected part of lightfrom a probing region along the probing direction at the receiving pointand directing said reflected part of light to the detector so as togenerate a signal from the detector based on the received, reflectedlight. The signal processor is adapted to determine at least a firstvelocity component of the inflow from the signal generated by theoptical receiver part. The invention further relates to a method ofoperating a wind turbine comprising a number of blades including atleast a first wind turbine blade extending substantially radially from ahub on a main shaft having a substantially horizontal centre axis, theblades together with the hub constituting a rotor with a rotor plane,and which can be put into rotation by wind, and each blade having aninnermost part comprising a root section of the blade and an outermostpart comprising a tip section of the blade.

BACKGROUND

Modern wind turbines are used to produce electricity. They are oftenvery large structures with blades of up to and in excess of 60 metersand made from fibre-reinforced polymer structures, such as shellelements. These wind turbines are provided with control devices whichmay prevent an overloading of the wind turbine and the blades at windgusts and high wind speeds. Such control devices can also be used toslowing the rotor down and bringing it to a complete halt, if the windspeed becomes too high. In addition to these devices the turbine maycomprise a braking system arranged in communication with the main shaftof the wind turbine.

The control devices may be formed of pitch-controlled blades mountedsuch on the hub that they are able to turn about their longitudinalaxis. The blades may thus be continuously adjusted to provide the liftrendering the desired power. In so-called stall-controlled wind turbinesthe blades are fixedly mounted on the hub and thus unable to turn abouttheir longitudinal axis. In this case, the stall properties of theblades are used to reduce the aerodynamic lift and thus the poweroutput.

The lengths of wind turbine blades have increased over the years and maynow as previously mentioned exceed 60 meters. The increase in lengthalso leads to increased mechanical loads from strong winds and fromfluctuations in the wind. The loads are primarily caused by changes inthe local inflow or turbulence. This in turn causes pressure changesover the surface of the wind turbine blade, which finally changes theloads on the blade. Typically, the loads are measured by use of straingauges, which are mounted on the blade or imbedded in the shellstructure of such a blade. Such strain gauges may for instance beresistive or in form of optical fibres, e.g. provided with Bragggratings. However, once the effect on the load is detected, it isalready too late to fully compensate for the load changes. To do so,information on the changes in the inflow or turbulence is neededbeforehand, i.e. before these inflow changes impact the wind turbineblade. This may for instance be obtained by arranging pitot tubes at theleading edge of the blade in order to probe the wind velocity. However,such pitot tubes influence the flow characteristics of the blade, andfurthermore pitot tubes may act as a lightning receptor, thus attractinglightning strikes potentially damaging the wind turbine blade. LightDetection And Ranging (LIDAR) systems may be used for non-invasiveprobing of wind velocities upwind of the wind turbine and have beenproposed used in connection with compensating for yaw errors or keepingthe rotational speed of the rotor substantially constant by pitching theindividual wind turbine blades. The LIDAR system is typically proposedto be mounted on top of the nacelle of the wind turbine and probes windspeeds in a probing region located hundreds of meters in front of thewind turbine.

U.S. Pat. No. 6,320,272 describes a wind turbine provided with a LIDARsystem on top of the nacelle. The LIDAR system is utilised foranticipating the wind speed upwind of the wind turbine and pitching theblade in order to obtain a substantially constant rotational speed ofthe rotor.

US2006140764 discloses a LIDAR system mounted in the hub of a windturbine. The LIDAR has a viewing direction, which is inclined to therotational axis so that the rotation of the hub ensures a scanning infront of the rotor.

US 2007075546 discloses a wind turbine provided with a LIDAR system formeasuring wind speeds in front of a portion of a wind turbine blade. TheLIDAR is mounted in the hub or at a base of the tower.

However, the wind is non-uniform over the length of a wind turbine bladedue to turbulence, tower shadow, wind shear, yaw errors, wake effectsand the like. This non-uniformity causes varying forces along theblades, which is turn cause fatigue loads and extreme loads on the windturbine. These phenomena become even more pronounced as the wind turbineblades become longer and longer. To compensate for such fluctuations itis not sufficient to obtain a single measurement hundreds of meters infront of the rotor.

WO2007045940 discloses a wind turbine blade having a variableaerodynamic profile. The document further mentions that a laseranemometer may be used to measure the wind speed in front of the bladeand that an anemometer may be arranged near the tip of the blade.However, the document does not provide any details on how such ananemometer should be mounted to the blade and where exactly theanemometer should probe the wind speed.

WO2004075681 discloses a method of controlling aerodynamic load of awind turbine based on a local blade flow measurement. The documentmentions that a laser Doppler anemometer may be utilised to measure theinstant angle of attack or the wind velocity. However, the document doesnot provide any details on where and how to arrange the anemometer.

DISCLOSURE OF THE INVENTION

It is an object of the invention to obtain a new blade, and whichovercomes or ameliorates at least one of the disadvantages of the priorart or which provides a useful alternative.

According to a first aspect of the invention, the emission point of theoptical transmitter part is located in the first blade at a first radialdistance from the centre axis, and the receiving point of the opticaltransmitter part is located in the first blade at a second radialdistance from the centre axis. In this way, the wind turbine facilitatesan optical measurement system, such as a LIDAR (Light Detection AndRanging) system, for measuring at least a first parameter of an inflow,such as wind speed or wind direction, locally at the first wind turbineblade. The optical emitting means and receiving means together define aprobing region or probing volume, in which a measurement may be made.

Throughout this document, inflow is to be understood as the apparentwind direction as seen from a point on the first blade, i.e. as thevector difference of the wind velocity vector and the local relativerotor velocity vector in the particular cross-section on the firstblade.

A reflected part of the light emitted from the optical emitting means isto be understood as any part of the light which returns towards thereceiving means by reflection on aerosols, by diffraction, by elastic orinelastic scattering, or by any other physical phenomenon. As such, itis not to be restricted to pure reflection in an optical meaning. In apreferred embodiment, the wind turbine comprises two or three blades.Preferably, the wind turbine is an upwind wind turbine with asubstantially horizontal shaft. According to an advantageous embodiment,the probing direction is directed in an upwind direction of the firstwind turbine blade.

In an advantageous embodiment, the emission point substantially flusheswith a surface of the first wind turbine blade. Thus, the opticalmeasurement system is truly obstruction free, since no protrusions orindentations are found on the blade surface, and only light is sent tothe probing region, thus not influencing the flow around the blade.

According to an advantageous embodiment, the first radial position issubstantially identical to the second radial position. Thus, thereflected light is collected substantially at the same radial positionas it is emitted.

According to a preferred embodiment, the first blade further comprisesadjustable flow altering means, such as distributed actuators, flaps ormicro tabs, for adjusting an aerodynamic parameter of the blade andlocated in a third radial distance from the hub, the adjustable flowaltering means being controlled by a controlling means, and wherein thecontrolling means are adapted to receive a signal from the signalprocessor, the signal being based on at least the first velocitycomponent. Thus, the flow altering means are adjusted in response to ameasured wind velocity component, the wind turbine thereby being able toadjust for fluctuations in the wind velocity. Advantageously, the thirdradial position is substantially identical to the first radial position.Thus, the flow altering means are adjusted locally in accordance with alocal wind velocity measurement.

In another embodiment, the wind turbine comprises a second wind turbineblade, and wherein the second wind turbine blade is provided withadjustable flow altering means, such as distributed actuator, flaps ormicro tabs, for adjusting an aerodynamic parameter of the second blade,the adjustable flow altering means being controlled by a controllingmeans, and wherein the controlling means are adapted to receive a signalfrom the signal processor, the signal being based on at least the firstvelocity component. Thus, the aerodynamic parameter of the second bladeis adjusted in accordance with measurements carried out via the firstblade. Thus, the second blade can be adjusted accordingly beforeencountering the position of the first blade assumed at the time ofcarrying out the wind velocity measurement. It is clear that the flowaltering means may be arranged in a radial distance corresponding to thefirst (or second) radial distance from the hub. Thus, the flow alteringmeans of the second blade are positioned approximately at the samedistance from the hub as the measurement carried out from the firstblade.

In an advantageous embodiment, the optical measurement system is adaptedfor probing the velocity component in a range of 0.5-10 m, or 0.75-8 m,or 1-5 m from the emission point. Thus, it is clear that it is indeed alocal, near field wind velocity, which is measured and that the flowaltering means are to be adjusted within tenths of seconds in order tocompensate for fluctuations.

In yet another advantageous embodiment, the emission point and/or thereceiving point of the first wind turbine blade is located between aleading edge of the first blade and a point of maximum thickness on apressure side of the blade. The emission point and/or the receivingpoint may for instance be located in vicinity of the leading edge of thefirst blade. The emission point and/or the receiving point may also belocated on a pressure side of the first wind turbine blade. Thereby, itis ensured that the probing direction is set substantially in an upwinddirection as seen from the blade profile.

In a first embodiment, the emission point during rotation of the rotorfollows a concentric circle having a radius corresponding to the firstradial distance from the centre axis, and wherein the probing directionis substantially arranged tangentially to the concentric circle. Thus,the probing beam is emitted substantially tangentially from a concentriccircle located at the first radial distance from the centre axis. In asecond embodiment, the emission point during rotation of the rotorfollows a concentric circle having a radius corresponding to the firstradial distance from the centre axis, and wherein the optical system isadapted to probe wind speeds in a probing volume located substantiallyat the first radial distance from the centre axis. Thus, the opticalsystem may probe wind speeds from a region on the same concentric circleor from another region located on an additional concentric circle havinga radius corresponding to the first radial distance from the centreaxis, the additional concentric circle for instance being located upwindof the rotor plane.

In an advantageous embodiment, the probing direction lies in a quadrantbetween a chord direction, seen from the leading edge of the blade, anda normal perpendicular to said chord direction and extending from thepressure side of the blade. Thus, the probing direction is set forwardof the leading edge of the blade and/or forward of the pressure side ofthe blade. If the chord direction is defined as 0 degrees and the normalas 90 degrees, the probing direction will advantageously lie in aninterval from 0 to 60 degrees, or even more advantageously 0 to 45degrees. If more than one probing beam is used in a single cross-sectionof the blade, all the probing beams may advantageously be located withinthese intervals.

According to an advantageous embodiment, the optical measurement systemis adapted for probing wind speeds in a probing volume located in anupwind plane upwind of the rotor plane. Advantageously, the probingvolume is located in the upwind plane at the first radial distance fromthe centre axis. By properly choosing the distance between the upwindplane and the rotor plane, the optical measurement system may probe thewind velocity of particles or aerosols, which the blade will laterimpact. Thus, the flow altering means may be very accurately adjusted inorder to compensate for the velocity fluctuations of the wind impactingthe blade.

In one advantageous embodiment according to the invention, the lightsource is separated from the emission point, the light source beingoptically connected to the emission point by a light guiding means, suchas an optical fibre. In this way, a single light source may convenientlysupply light to multiple light emitting means within the blade.Furthermore, the radial position of the light source is thus notconstrained to being substantially the first radial distance, but may bechosen more freely. Thus, the light emitting means may be located wherea light source would not physically fit, or where a light source wouldnot be able to function reliably, e.g. due to the mechanical influenceof the rotor rotation. Furthermore, it is easier to gain access to thelight source, e.g. if needing maintenance. In a preferred embodiment,the light guiding means is an optical fibre. In this way, the lightemitting means may be electrically isolated from the light source,thereby greatly reducing the risk of lightning strikes to the lightemitting means. The light source may advantageously be located in thehub or in a nacelle of the wind turbine. In this way, a single lightsource may conveniently be used to supply light to multiple lightemitting means located in different blades.

The light source or the light guiding means comprises beam splittingmeans, and wherein the beam splitting means are optically connected toboth the light guiding means and a second transmitter part having asecond emission point via a second light guiding means. Thus, a simplesolution for providing light to separate optical measurement systems inindividual blades or individual positions on the same blade is provided.Alternatively, the light source is connected to multiplexing means, inorder to supply the different optical transmitter parts with lightsequentially, i.e. a time slot for the first emitting means, then a timeslot for the second emitting means, etc. Thus, according to anadvantageous embodiment, the emission point is located in a first blade,and the second emission point is located in a second blade, i.e.different wind turbine blades. In another advantageous embodiment, theemission points are located in the same wind turbine blade so that thefirst emission point is located at a first radial distance from the hubor central axis, and the second emission point is located at a firstadditional radial distance from the hub or central axis.

In one embodiment, the transmitter part comprises a transmitter path foroutgoing light, and the receiver part comprises a receiver path forreceiving the reflected part of light, and wherein the transmitter pathand the receiver path have an overlapping part, wherein the transmitterpath and the receiver path are substantially overlapping. Thus, theoverlapping part may be used for both the transmitter part and thereceiver part of the optical measurement system, and in particular theoverlapping part may comprise the emission point and receiving point.Thus, the emission point and receiving point are coincident. Theemission point, such as a focusing lens, can thus also be used forcollecting the reflected light and guiding it to the detector.

Advantageously, a beam splitter is arranged in both the transmitter pathbetween a light source and the emission point and in the receiver pathbetween the receiving point and the detector. Thus, it is seen that thecommon path extends from the emission/receiving point to the beamsplitter.

According to a first advantageous embodiment, the optical measurementsystem is a laser Doppler anemometry (LDA) system. According to aparticularly advantageous embodiment, the LDA system is a Michelson typeanemometer. The aforementioned beam splitter can thus be used to directa part of the incoming light to a reference mirror, which is later mixedwith the received, reflected light from the particles or aerosols. Themeasured frequency shift corresponds to the wind velocity in the probingdirection. When using an LDA system mounted on or in a wind turbineblade, it should be noted that the Doppler shift occurs partly due tothe light source moving, i.e. the emission point rotating together withthe blade, and due to the movement of the particles or aerosolsreflecting the light. The measured Doppler shift corresponds to the windvelocity “observed” by the blade in the probing direction, i.e. theinflow which is a combination of the local rotor velocity and the windvelocity.

According to another embodiment, the optical measurement system is basedon feedback into the light source, i.e. the laser. Thus, at least a partof the received, reflected light is transmitted to the light source inorder to perturb the power output of the light source. Thus, thedetector detects the perturbed power output, and the wind velocity iscalculated from the perturbed power output.

It is advantageous to use a coherent light source, e.g. a laser. Thelaser may be a continuous wave laser or a pulsed laser. The laser mayfor instance be a CO2 laser, an Argon laser or a Nd:YAG laser. However,the laser may also be a laser diode or a VCSEL, which is particularlysuited for compact units. In principle, it may also be sufficient to useLEDs or OLEDs as far as the coherence of such light sources allows this.

The detectors may be any suited detector, such as a photoresistor, aphotomultiplier tube, a photo diode or the like. The signal processormay advantageously comprise a phase locked loop or a frequency lockedloop, thereby deriving for instance the Doppler shift of the wavelengthof the light source.

Light source means any light source being suited for probing windvelocities, advantageously a laser as previously mentioned. Thewavelength of the laser beam may lie in the ultraviolet range, thevisible range, or the infrared range. Thus, the wavelength may be any inthe range from e.g. 100 nm to 20 μm. However, the invention is notrestricted to these wavelengths.

According to an advantageous embodiment, the receiving point (or thereceiver part of the optical measurement system) has a direction of highsensitivity, and wherein the direction of high sensitivity is orientedto substantially coincide with the probing direction.

According to another advantageous embodiment, at least the transmitterpart, the receiver part and the detector are arranged in a single, firstunit in the first wind turbine blade. Preferably, the light sourceand/or the signal processor are also arranged in said first unit. Thus,the first unit may easily be inserted or replaced in the first windturbine blade. However, the first unit may also be provided with anincoupling for coupling in light from the light source and/or anoutcoupling for coupling out light and guiding said light to thedetector.

Advantageously, the first unit may be arranged in a bushing, such as asleeve tube, in the first wind turbine blade. The bushing may forinstance be moulded into the first wind turbine blade duringmanufacture. Thus, the first unit may easily be replaced. Furthermore,this means that optical fibres or other waveguides do not have to bemoulded into the structure during manufacture. Also, the bushing may bepre-arranged so as to set the desired probing direction. Thus, theoptics of the optical measurement system need not be adjusted afterinstalling it into the wind turbine. The bushing may also containadjustment means as to align the first unit according to the bladegeometry.

The bushing may for instance be provided as a sleeve tube. The sleevetube may for instance be provided with an inner thread, whereas thefirst unit may be provided with a mating outer thread. Alternatively,the first unit may be adhered to the sleeve tube or be mechanicallyengaged, e.g. by screws, nuts and bolts or the like. Using a sleeve tubewill make the optical fibres replacable and thereby ensure a maintenancefriendly system. Furthermore, the sleeve tube may be provided with asmall degree of adjustment possibilities in order to for instance adjustthe probing direction within a few degrees, e.g. up to two degrees.

In one embodiment, the first wind turbine blade further is provided witha cleaning system, using e.g. pressurised air, adapted to clear asurface of the emission point and/or the receiving point. Thus, thecleaning system can clean the optical measurement system, which overtime may become polluted with particles carried by the wind and due tothe rotation of the wind turbine blade. The cleaning system may forinstance be provided in connection with the sleeve tube.

According to an advantageous embodiment, the optical measurement systemis adapted to emit at least a first probing beam and a second probingbeam. This can for instance be achieved by letting the opticalmeasurement system comprise two separate transmitter/receiver units. Itcan also be achieved by splitting the light beam up into two separatebeams and emitting light from two separate emission points. Thus, thelight is also advantageously collected at two separate receiving points.However, it may also be possible to emit two or more laser beams fromthe same emission point, e.g. by use of an optical grating. The twoseparate beams may probe wind velocities in two different probingvolumes, advantageously located in vicinity of each other. Thus, theadjustment of the flow altering means can be carried out in accordancewith a weighting between two measurements, e.g. the average between thetwo measurements, thus compensating for local turbulence or windvelocity fluctuations.

In one embodiment, the first probing beam and the second probing beamform a probing angle lying in an interval of 5-90 degrees, oradvantageously 7-75 degrees, or advantageously 10-60 degrees. By probingin two different directions, it is possible to derive two velocitycomponents of wind speed inflow vector or correspondingly a wind speedin a plane and the angle of attack. By adding a third probing directionit may further be possible to derive a third velocity component.

In principle, it may also be possible to split light up into twoseparate probing beams which are emitted from two separate emissionpoints, and which cross each other in a common measurement volume orprobing volume. Thereby a fringe pattern may arise in the probingvolume, and the wind velocity can be measured by measuring the frequencyof wind particles passing through the common volume. However, thisembodiment demands for a high precision of the two separate probingbeams.

According to an advantageous embodiment, the first probing beam and thesecond probing beam are oriented substantially in a cross-sectionalplane of a local cross-section of the blade. Thus, the two beams areemitted in the same cross-sectional plane of the local blade profile.Thus, it is possible to derive the two velocity components in thecross-sectional plane, e.g. the wind velocity and the local rotorvelocity, thus being able to derive the exact inflow, such as wind speedand angle of attack. The cross sectional-plane is the plane, whichincludes both the local chord and the local camber.

However, the local rotor velocity can also be deduced from therotational speed of the rotor. Thus, two separate velocity componentsare not necessary. In this case, it may be more appropriate to use thetwo separate wind velocity measurements to calculate the average betweenthe two measurements from the two probing regions. In this case, thevelocity measurements of course have to compensate for the mutualprobing angle.

When using the two probing beams for deriving two separate wind velocitycomponents it may be advantageous to use a large angle between the twoprobing beam, ideally 90 degrees. However, due to constructional reasonsthe probing angle may advantageously be lower, e.g. 45-60 degrees. Whenusing two probing beams for deriving an average of wind velocities fromtwo probing volumes in vicinity of each other, it is advantageous to usea low probing angle, e.g. 5-30 degrees.

When needing to derive two separate wind velocity components, it mayalso in principle be possible to use a single probing beam and twoobservation directions, i.e. via a single emission point and tworeceiving points. The angle between the two observation directions canthus be utilised to derive the two velocity components. However, it isdifficult to achieve a sufficiently large angle between the observationdirections, since the probing volume must either be located very closeto the wind turbine blade or the two receiving points be spaced farapart, in which case also collection of the reflected light may beproblematic.

According to one embodiment, the first wind turbine blade comprises aprofiled contour, which in the radial direction is divided into a rootregion with a substantially circular or elliptical profile closest tothe hub, an airfoil region with a lift generating profile farthest fromthe hub, and preferably a transition region between the root region andthe airfoil region, the transition region having a profile graduallychanging in the radial direction from the circular or elliptical profileof the root region to the lift generating profile of the airfoil region.Thus, the wind turbine blade has a per se conventional profiled contour.

According to an advantageous embodiment, the emission point is locatedin the airfoil region. Preferably, the receiving point is also locatedin the airfoil region. Further, the flow altering means mayadvantageously also be located in the airfoil region. According to anadvantageous embodiment, the emission point and/or the receiving pointis located within an outer 75% of the airfoil region, i.e. the partfarthest from the hub. According to another advantageous embodiment, theemission point and/or the receiving point is located within an outer 50%of the airfoil region. The various emission points and receiving pointsmay be located within said outer regions only.

According to one advantageous embodiment, the first blade comprises aplurality of sets of emission points, said sets of emission points beinglocated at different radial distances from the centre axis. Each set mayadvantageously comprise one, two or three emission points. Each set ofemission points corresponds to separate flow altering devices. Thus, anumber of local means are provided to control the local aerodynamicperformance and alleviating of loads.

The optical systems may be powered by local power supply units. Thepower supply units may for instance be located within the hub or thenacelle. In one embodiment, the local power supply is located within thefirst wind turbine blade. Such a power supply may for instance drawenergy from mass and gravity variations due to the rotation of therotor.

According to another advantageous embodiment, an additional opticalsystem is provided for probing upwind wind speeds in front of the rotor.The additional optical system may for instance be installed on top ofthe nacelle of the wind turbine or in the hub. This system can be usedfor compensating for yaw errors, wind shear or the like or for ensuringa substantially constant rotational speed of the rotor. This may beobtained by pitching the individual blades, e.g. cyclic pitching of theblades. Thus, the invention provides an optical system for compensatingfor overall wind fluctuations and reacting to these fluctuations bypitching the blades and optical systems for probing local windfluctuations in vicinity of the wind turbine blades, these localfluctuations being compensated for by the local flow altering means.

According to one advantageous embodiment, the first wind turbine bladehas a blade length (L), and the emission point and the receiving pointare located within a blade length interval of 0.2L to 0.9L, oradvantageously within a blade length interval of 0.22L to 0.85L, or moreadvantageously within a blade length interval of 0.25L to 0.8L, as seenfrom the root of the first blade. In this notation, the blade root islocated at 0 position and the blade tip at position L. Thereby, thesystem is readily adapted to probe wind speeds in front of the blade atthe radial positions of the blade contributing most to the overallenergy production of the wind turbine.

According to another advantageous embodiment, the first wind turbineblade has a blade length (L), and the probing region is located at aposition in which the wind impacts the first wind turbine blade or asecond wind turbine blade within a blade length interval of 0.5L to0.9L, or advantageously within a blade length interval of 0.55L to0.80L, or more advantageously within a blade length interval of 0.6L to0.75L, as seen from the root of the first blade. According to yetanother advantageous embodiment, the emission point is located withinthe same blade length interval. Thereby, the optical measurement systemmay be adapted to probe the region in which the blade has its largestloads and where compensation has its largest effect on loadfluctuations.

In an advantageous embodiment, the first wind turbine blade ispitchable, and the optical measurement system comprises compensationmeans for compensating for a pitch angle of the first blade. In a firstsimple embodiment, the compensation means may simply be computationalmeans, which compensate the wind velocity measurement in dependence onthe pitch angle of the first wind turbine blade. Computational means mayalso be used for compensating for variations in the rotational speed ofthe rotor, thus influencing the local angle of attack and wind velocityperceived by a radial blade section.

According to another advantageous embodiment, the first wind turbineblade is pitchable, and the probing direction is variable in dependenceon a pitch angle of the first blade. Thereby, it is for instancepossible to adjust the probing direction so that the probing region doesnot change despite of the blade pitch being changed and/or in order tomaximise the resolution of the probed wind speeds. Similarly, theprobing direction may be variable in dependence on a rotational speed ofthe rotor.

It is recognised that the probing direction may be varied in a lot ofdifferent ways. The majority of the transmission part of the opticalmeasurement system may for instance be contained in a single unit, andwhere this unit is variable in angle in relation to the first windturbine blade. In another embodiment, the optical measurement system isadapted to vary a position of incoming light on a transmitting lens. Thetransmitting lens may for instance be a lens located at the emissionpoint of the optical measurement system, and the position of incominglight may for instance be varied in angle or position by changing theposition of the light source. In one embodiment this is obtained bymoving the light source itself, and in another embodiment, this isobtained by moving the position of the emission end of an optical fibre.In an alternative or supplementary embodiment, the optical system isadapted to vary a position of a transmitting lens. Thereby, the lens maybe moved for instance in a substantially transverse direction of theincoming light.

According to a second aspect, the invention provides a method, whereinthe method comprises the steps of: a) emitting light in a probingdirection from an emission point on the first wind turbine blade, saidemission point being located in a first radial distance from the centreaxis, b) receiving a reflected part of light from a probing region alongthe probing direction at a receiving point located on the first windturbine blade at a location in a second radial distance from the centreaxis, c) directing said reflected part of light to a detector, d)generating a signal based on detected light in step c), and e)calculating a first velocity component based on the signal from step d).As previously mention, the second radial distance preferably correspondsto the first radial distance.

In one advantageous embodiment, the method further comprises the stepof: f) adjusting adjustable flow altering means on the first windturbine blade in order to adjust an aerodynamic characteristics of thefirst wind turbine blade, the adjustable flow altering means beinglocated at a third radial distance from the centre axis. As previouslymentioned, the third radial distance preferably corresponds to the firstradial distance.

In another advantageous embodiment, the method further comprises thestep of adjusting the probing direction in dependence on a pitch angleof the first wind turbine blade and/or a rotational velocity of therotor.

The method may of course also apply to any of the aforementionedembodiments of the wind turbine.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in detail below with reference to anembodiment shown in the drawings, in which

FIG. 1 shows a wind turbine,

FIG. 2 shows a schematic view of an airfoil profile,

FIG. 3 shows a schematic view of flow velocities and aerodynamic forcesat an airfoil profile,

FIG. 4 shows a schematic view of a wind turbine blade provided withlocal optical measurement systems and corresponding local flow alteringmeans,

FIGS. 5 a-c show cross-sectional views of embodiments with differentarrangements of the local measurement systems,

FIGS. 6 a-d show schematic views of rotors with different probingdirections,

FIG. 7 shows a schematic view of a wind turbine where the opticalmeasurement system probes wind speeds in a probing region located infront of the rotor plane,

FIG. 8 shows a schematic view of a wind turbine rotor provided with acentrally located laser source,

FIGS. 9 a and 9 b show schematic views of a first and a secondembodiment of a laser Doppler anemometry system, respectively,

FIG. 10 shows a schematic view of a laser Doppler anemometry systemcomprising two probing beams,

FIGS. 11 a-g show various embodiments of flow altering devices, and

FIG. 12 shows a first embodiment, where the probing direction isvariable,

FIG. 13 shows a first embodiment, where the probing direction isvariable, and

FIG. 14 shows a first embodiment, where the probing direction isvariable.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a conventional modern upwind horizontal axis windturbine according to the so-called “Danish concept” with a tower 4, anacelle 6 and a rotor with a substantially horizontal rotor shaft. Therotor includes a hub 8 and three blades 10 extending radially from thehub 8, each having a blade root 16 nearest the hub and a blade tip 14furthest from the hub 8.

FIG. 2 shows a schematic view of an airfoil profile 50 of a typicalblade of a wind turbine depicted with the various parameters, which aretypically used to define the geometrical shape of an airfoil. Theairfoil profile 50 has a pressure side 52 and a suction side 54, whichduring use—i.e. during rotation of the rotor—normally face towards thewindward side and the leeward side, respectively. The airfoil 50 has achord 60 with a chord length c extending between a leading edge 56 and atrailing edge 58 of the blade. The airfoil 50 has a thickness t, whichis defined as the distance between the pressure side 52 and the suctionside 54. The thickness t of the airfoil varies along the chord 60. Thedeviation from a symmetrical profile is given by a camber line 62, whichis a median line through the airfoil profile 50. The median line can befound by drawing inscribed circles from the leading edge 56 to thetrailing edge 58. The median line follows the centres of these inscribedcircles and the deviation or distance from the chord 60 is called thecamber f. The asymmetry can also be defined by use of parameters calledthe upper camber and lower camber, which are defined as the distancesfrom the chord 60 and the suction side 54 and pressure side 52,respectively.

Airfoil profiles are often characterised by the following parameters:the chord length c, the maximum camber f, the position d_(f) of themaximum camber f, the maximum airfoil thickness t, which is the largestdiameter of the inscribed circles along the median camber line 62, theposition d_(t) of the maximum thickness t, and a nose radius (notshown). These parameters are typically defined as ratios to the chordlength c.

FIG. 3 shows a schematic view of flow velocities and aerodynamic forcesat the airfoil profile 50. The airfoil profile is located at the radialposition or radius r of the rotor of which the blade is part, and theprofile is set to a given twist or pitch angle θ. An axial free streamvelocity v_(a), which according to theory optimally is given as ⅔ of thewind velocity v_(w), and a tangential velocity r·ω, which is oriented ina direction of rotation 64 for the rotor, combined form a resultantvelocity v_(r). Together with the chord line 60, the resultant velocityv_(r) defines an inflow angle, φ, from which an angle of attack α can bededucted.

When the airfoil profile 50 is impacted by an incident airflow, a lift66 is generated perpendicular to the resultant velocity v_(r). At thesame time, the airfoil is affected by a drag 68 oriented in thedirection of the resultant velocity v_(r). Knowing the lift and drag foreach radial position makes it possible to calculate the distribution ofresultant aerodynamic forces 70 along the entire length of the blade.These aerodynamic forces 70 are typically divided into two components,viz. a tangential force 74 distribution (in the rotational plane of therotor) and a thrust 72 oriented in a right angle to the tangential force74. Further, the airfoil is affected by a moment coefficient 75.

The driving torque of the rotor can be calculated by integrating thetangential force 74 over the entire radial length of the blade. Thedriving torque together with the rotational velocity of the rotorprovides the overall rotor power for the wind turbine. Integrating thelocal thrust 72 over the entire length of the blade yields the totalrotor thrust, e.g. in relation to the tower.

If the wind speed changes or local wind speed fluctuations occur, thevelocity triangle is influenced and hence also the lift and the forces(or loads) influencing the blade profile.

The load fluctuations can be alleviated by using active flow alteringdevices, which for instance may change the overall camber of the localprofile or which may alter the lift coefficient, thereby readjusting thevelocity triangle (v_(r), v_(a), r·ω) and the force triangle (70, 72,74). However, in order to do so, information about the wind speedchanges or fluctuations need to be known before the flow actuallyimpacts the local blade profile 50 in order to compensate quicklyenough.

FIG. 4 shows a schematic view of a first embodiment of a wind turbineblade 10 according to the invention. The wind turbine blade 10 has theshape of a conventional wind turbine blade and comprises a root region30 closest to the hub, a profiled or an airfoil region 34 furthest awayfrom the hub and a transition region 32 between the root region 30 andthe airfoil region 34. The blade 10 comprises a leading edge 18 facingthe direction of rotation of the blade 10 when the blade is mounted onthe hub, and a trailing edge 20 facing the opposite direction of theleading edge 18.

The airfoil region 34 (also called the profiled region) has an ideal oralmost ideal blade shape with respect to generating lift, whereas theroot region 30 due to structural considerations has a substantiallycircular or elliptical cross-section, which for instance makes it easierand safer to mount the blade 10 to the hub. The diameter (or the chord)of the root region 30 is typically constant along the entire root area30. The transition region 32 has a transitional profile 42 graduallychanging from the circular or elliptical shape 40 of the root region 30to the airfoil profile 50 of the airfoil region 34. The width of thetransition region 32 typically increases substantially linearly withincreasing distance r from the hub.

The airfoil region 34 has an airfoil profile 50 with a chord extendingbetween the leading edge 18 and the trailing edge 20 of the blade 10.The width of the chord decreases with increasing distance r from thehub.

It should be noted that the chords of different sections of the bladenormally do not lie in a common plane, since the blade may be twistedand/or curved (i.e. pre-bent), thus providing the chord plane with acorrespondingly twisted and/or curved course, this being most often thecase in order to compensate for the local velocity of the blade beingdependent on the radius from the hub.

The wind turbine blade 10 according to the invention is provided with afirst optical measurement system or laser Doppler anemometer (LDA)system 80, which when the wind turbine blade 10 is mounted to the hub islocated at a first radial distance r₁ from a central axis of the rotor15 and thus also in a first distance from the hub. The wind turbineblade is further provided with a second laser Doppler anemometer system81 located at a second radial distance r₂ from the central axis of therotor as well as a third laser Doppler anemometer system 82 located at asecond radial distance r₂ from the central axis of the rotor. The threelaser Doppler systems 80, 81, 82 are operationally connected to a firstflow altering device 90, a second flow altering device 91, and a thirdflow altering device 92, respectively. In the depicted embodiment, theflow altering devices 90, 91, 92 are surface mounted flaps, which can bedeployed in accordance with the wind velocity measurement obtained bythe laser Doppler systems 80, 81, 82 in order to adjust the velocitytriangle and load triangle as described in relation to FIG. 3. Each LDAsystem 80, 81, 82 may comprise one, two or three probing beams formeasuring wind velocities in one, two or three probing volumes,respectively.

FIG. 5 a-c show various embodiments of wind turbine blades provided withLDA systems in different configurations. For the sake of clarity, theflow altering devices are not depicted in these figures.

FIG. 5 a shows a cross-sectional view of a first embodiment of a windturbine blade 110 provided with an LDA system 180. The LDA system 180 isarranged with an emission point 185 near the leading edge of the blade.The LDA system 180 emits a probing beam 186 along a probing directionand probes wind speeds in a probing region or a probing volume 187located substantially upwind of the wind turbine blade cross-section.The probing beam 186 is focused in a distance f_(L) from the emissionpoint 185. The probing region 187 is here depicted as beingsubstantially spherical. However, in practise the probing region, whichnormally is defined as the region of the full width at half maximum(FWHM) intensity, is elongated in the probing region. The probing regionis typically longer, when the focal length f_(L) is increased. In thisembodiment, the probing direction is set in a direction, where theprobing beam is emitted in a direction from the leading edge andslightly towards the pressure side of the blade 110. Particles oraerosols passing through the probing region 187 backscatter or reflectlight. This reflected light is collected by the LDA system 180 at areceiving point 185. Usually, the emission point and receiving point arecoincident, e.g. delimited by a window or a lens, which is used for bothfocusing the probing beam 186 and collecting the reflected light.

FIG. 5 b shows a cross-sectional view of a second embodiment of a windturbine blade 210 in which like numerals refer to like parts of thefirst embodiment shown in FIG. 5 a. In this particular embodiment, thewind turbine blade 210 is provided with two LDA systems 280, 280′ in thesame cross-section. A first LDA system 280 is arranged with an emissionpoint 285 near the leading edge of the blade. The first LDA system 280emits a probing beam 286 along a probing direction directed from theleading edge and oriented substantially in continuation of a chord 260of the local cross-sectional profile and probes wind speeds in a probingregion or a probing volume 287 located in front of the leading edge ofthe profile. A second LDA system 280′ is arranged with an emission point285′ on the pressure side of the profile. The second LDA system 280′emits a probing beam 286′ along a probing direction and probes windspeeds in a probing region or a probing volume 287′ located in front ofthe leading edge of the profile and on the pressure side of the blade210, i.e. typically from a plane upwind of the rotor plane. The anglebetween the two probing beams 286, 286′ is denoted δ.

It is seen that both emission points 285, 285′ are located in a regionbetween the leading edge and the position of maximum thickness on thepressure side of the blade, cf. also definitions given in relation toFIG. 2. In the coordination system, where the emission direction fromthe leading edge in direct continuation of the chord 260 is defined as 0degrees and a normal 288 to the chord on the pressure side of the bladeis defined as 90 degrees, the emission point and probing direction areadvantageously found in the quadrant between 0 and 90 degrees. Moreadvantageously, the probing direction is found between 0 and 60 degrees,or even more advantageously 0 to 45 degrees.

The embodiments shown in FIGS. 5 a and 5 b are used for probing upwindwind speeds of the local cross-sectional profile of the first windturbine blade. Based on these measurements, local flow guiding devicesof the first wind turbine blade (not shown) are controlled. The LDAsystems are advantageously adapted for probing the velocity component ina probing region located in the range of 0.5-10 m, or 0.75-8 m, or 1-5 mfrom the emission points. Thus, the systems indeed probe local windspeeds and wind fluctuations.

However, wind speed measurements can also be used to control flowguiding devices of a second wind turbine blade. In this situation, itmay be desired to probe wind speeds in probing regions located upwind ofthe second wind turbine blade instead. An example of such an embodimentis depicted in FIG. 5 c showing the cross-sectional profile of a windturbine blade 910. A first LDA system 980 is arranged with an emissionpoint 985 near the trailing edge of the blade. The first LDA system 980emits a probing beam 986 along a probing direction directed from thetrailing edge and oriented substantially in continuation of a chord 260of the local cross-sectional profile and probes wind speeds in a probingregion or a probing volume 987 located behind of the leading edge of theprofile. A second LDA system 980′ is arranged with an emission point985′ on the pressure side of the profile. The second LDA system 980′emits a probing beam 986′ along a probing direction and probes windspeeds in a probing region or a probing volume 987′ located behind thetrailing edge of the profile and on the pressure side of the blade 910,i.e. typically from a plane upwind of the rotor plane. The angle betweenthe two probing beams 986, 986′ is denoted δ. In this embodiment, theprobing range f_(L) may advantageously be larger than the probing rangeof the embodiments shown in FIGS. 5 a and 5 b. The probing regions 987,987′ may advantageously be located within range of 0.5 m to 15 m of thesecond blade.

FIGS. 6 a-d show various possible probing directions for a LDA systemlocated in a wind turbine blade.

FIG. 6 a shows a first embodiment illustrating a possible probingdirection of the system. During rotation of the rotor a LDA system 380located within a wind turbine blade of the rotor follows a motion alonga concentric circle 325 having a radius from a central axis of the rotor315. In this embodiment, the LDA system 380 emits a probing beam (orbeams) in the cross-sectional plane of the local profile at the LDAsystem 380. Thus, the probing beams are directed tangentially to theconcentric circle 325. The probing region is located at a second radiusfrom the central axis on a second concentric circle 325′. Thus, the partof the blade, which in fact impacts the wind particles in the probingregion may be located at this second radius on the blade. Thus, the flowaltering devices may advantageously be located at this second radius orbe displaced towards the second radius.

FIG. 6 b shows a second embodiment illustrating a possible probingdirection of the system. During rotation of the rotor a LDA system 480located within a wind turbine blade of the rotor follows a motion alonga concentric circle 425 having a radius from a central axis of the rotor415. In this embodiment, the LDA system 480 emits a probing beam (orbeams) in an inwards direction out of the cross-sectional plane of thelocal profile at the LDA system 480. The probing region is in thisembodiment located at the same radial distance from the central axis 415as the emission point. Thus, the system may more accurately probe thewind speeds of particles, which the local blade section in fact impacts.

FIG. 6 c shows a third embodiment illustrating a possible probingdirection of the system corresponding to the embodiment shown in FIG. 5c. During rotation of the rotor an LDA system 1080 located within afirst wind turbine blade 1010 of the rotor follows a motion along aconcentric circle 1025 having a radius from a central axis of the rotor1015. In this embodiment, the LDA system 1080 emits a probing beam (orbeams) in an inwards direction out of the cross-sectional plane of thelocal profile at the LDA system 1080. The probing beam(s) are emittedfrom a location in vicinity of the trailing edge of the first blade 1010and the probing region is located in front of the leading edge of asecond wind turbine blade 1010′. The probing region is in thisembodiment located in the same radial distance from the central axis1015 as the LDA system 1080. The probing direction may also besubstantially tangentially to the concentric circle 1025.

FIG. 6 d shows a fourth embodiment illustrating a possible probingdirection of the system. During rotation of the rotor a LDA systemlocated within a wind turbine blade of the rotor follows a motion alonga first concentric circle, whereas the probing region is located at asecond concentric circle having a radius smaller than the firstconcentric circle. In the illustrated embodiment, the observationdirection and the probing region are located so that the observationdirection is directed substantially tangentially to the secondconcentric circle. However, it is recognised that the LDA system may belocated further inboard or further outboard of the blade.

The LDA system may advantageously probe wind speeds from a probingregion located upwind of, i.e. in front of, the rotor plane. Thissituation is depicted in FIG. 7. During rotation of the rotor, an LDAsystem located within a first wind turbine blade of the rotor follows amotion along a concentric circle within a rotor plane 1125. The probingregion is located in a second plane 1127 located upwind of the rotorplane.

In the previously shown embodiments, the LDA systems are depicted as asingle unit within the blade. However, embodiments where the lightsource, i.e. the laser source, is located within the hub or within thenacelle of the wind turbine are also contemplated. Such an embodiment isdepicted in FIG. 8. One or more laser sources 585 are located within thehub of a rotor. Laser light from the laser source is split up into anumber of separate beams, which are directed to emission points withinthe wind turbine blades of the rotor, e.g. via optical fibres. One beamis split up into a number of separate beams by a beam splitter 586 oralternatively a multiplexing unit. The split up light is guided to afirst LDA unit 580 via a first optical fibre 587, to a second LDA unit581 via a second optical fibre 588, and a third LDA unit 582 via a thirdoptical fibre 589. In this embodiment, the LDA units 580, 581, 582 emitprobing beams (not shown) from an emission point in vicinity of theleading edge of the wind turbine blade. The wind speeds measured by thefirst LDA unit 580 are used for controlling a first flow altering device590, the wind speeds measured by the second LDA 581 unit are used forcontrolling a second flow altering device 591, and the wind speedsmeasured by the third LDA 581 unit are used for controlling a third flowaltering device 592 in order to alleviate for local load fluctuations.

FIG. 9 a shows a first embodiment of an LDA unit 680 usable for theinvention. The LDA unit 680 comprises a light source means 674 includingfor instance a laser diode and a condensing lens. The light emitted fromthe light source means 674 is directed to a beam splitter 675, whichsplits the light up into a reference beam, which is guided to areference mirror 676, and a probing beam, which is sent through a lenssystem 677 and optionally a window 678, which thus constitutes theemission point of the LDA unit 680. In an alternative embodiment, a lensof the lens system 677 may constitute the emission point. Lightreflected by particles or aerosols passing through the probing volume isreflected or backscattered and collected through the window 678, whichthen passes through the lens system 677 and to the beam splitter 675,where the reflected light is mixed with the reference beam. The mixedlight is detected by a photo detector 679. It is seen that the systemcorresponds to a Michelson based laser Doppler anemometry system, wherethe detected Doppler shift depends on the velocity of the particlespassing through the probing volume. The signal from the photo detector679 is sent to an amplifier 693, and from the amplifier 693 on to asignal processor 694, e.g. comprising a phase locked loop or a frequencylocked loop. The signal from the signal processor 694 is sent to anelectrical output 695, which can be used for controlling thecorresponding flow altering devices.

FIG. 9 b shows a second embodiment of an LDA unit 780 usable for theinvention and wherein like numerals correspond to like parts of the LDAunit shown in FIG. 9 a. The LDA unit 780 comprises a light source means774 including an incoupling for light from a central laser, e.g. asshown in FIG. 8, and a condensing lens. The light emitted from the lightsource means 774 is directed to a beam splitter 775, which splits thelight up into a reference beam, which is guided to a reference mirror776, and a probing beam, which is sent through a lens system 777 andoptionally a window 778, which thus constitutes the emission point ofthe LDA unit 780. In an alternative embodiment, a lens of the lenssystem 777 may constitute the emission point. Light reflected byparticles or aerosols passing through the probing volume is reflected orbackscattered and collected through the window 778, which then passesthrough the lens system 777 and to the beam splitter 775, where thereflected light is mixed with the reference beam. The mixed light isdetected by a photo detector 779. It is seen that the system correspondsto a Michelson based laser Doppler anemometry system, where the detectedDoppler shift depends on the velocity of the particles passing throughthe probing volume. The signal from the photo detector 779 is sent to anamplifier 793, and from the amplifier 793 on to a signal processor 794,e.g. comprising a phase locked loop or a frequency locked loop. Thesignal from the signal processor 794 is sent to an electrical output795, which can be used for controlling the corresponding flow alteringdevices.

As previously mentioned, FIGS. 5 a and 5 b show embodiments using twoprobe beams in the same cross-sectional profile of the wind turbineblade, and where the probe beams are generated by separate LDA units.However, it is also possible to generate two or more probe beams from asingle LDA unit. Such an embodiment is shown in FIG. 10. In thisembodiment an incoming beam is split up into two separate beams, e.g. byuse of a Wollaston prism 896, thus generating a first beam and a secondbeam. The first and the second beam may advantageously each be sent to asecond beam splitter 875, which splits the beam up into a referencebeam, which is sent to a reference mirror 876, and a probe beam, whichmay be sent through a lens system and to the emission point (now shown).In an alternative embodiment, the incoming beam is split up intoseparate beams by a grating. If two probe beams are needed it may forinstance be possible to use the zero'th order beam and one of the firstorder beams and suppress the other orders or alternatively using a firstorder and a second order beam and suppressing the rest. If three probebeams are needed it may be possible to use the zero'th order beam andboth first order beams.

FIGS. 11 a-g show various embodiments of flow altering devices suitablefor the invention. Flaps are one type of flow altering means, which arevery suitable for fast adjustments of the aerodynamic properties of thelocal profile. Flaps may be implemented in various ways. As shown inFIG. 11 a, the flaps may be implemented as surface mounted flaps, whichwhen deployed, protrude from the surface of the blade profile. A flapmay also be provided as a separate element as shown in FIG. 11 b, whichmay be moved rotational and/or translational in relation to the bladeitself. Thus, the blade profile is a multi element profile.Alternatively, the flap may be implemented as a camber flap as shown inFIG. 11 c, which can be used to change the camber line of the bladeprofile. It is also possible to use micro tabs as shown in FIG. 11 dplaced either on the upper and/or lower surfaces of the local profile.Such flow altering devices may very quickly be deployed so that theyprotrude from the surface of the blade.

The flow altering means may also comprise of a number of ventilationholes for blowing or suction between an interior of the blade and anexterior of the blade. The ventilation holes are advantageously appliedto the suction side of the blade as shown in FIGS. 11 e and 11 f. Theventilation holes can be utilised to create a belt of attached flow. Airvented from the ventilation holes may be used to energise andre-energise the boundary layer in order to maintain the flow attached tothe exterior surface of the blade as shown in FIG. 11 f. Alternatively,the ventilation holes may be used for suction as shown in FIG. 11 e,whereby the low momentum flow in the boundary layer is removed and theremaining flow thereby re-energised and drawn towards the surface of theblade.

It is also possible to use a slat as shown in FIG. 11 g. The slat may beconnected to the blade in such a way that it is rotational and/ortranslational movable in relation to the local blade profile.

FIG. 12 shows a cross-sectional view of a first embodiment of a windturbine blade 1210 provided with an LDA system 1280 having a variableprobing direction. In this embodiment, the LDA system 1280 is variablein relation to the local blade section. Thereby, a position of a probingvolume 1287 may also be varied in relation to the local blade section.Thereby, it is possible to compensate for either a change in blade pitchangle and/or the rotational speed of the rotor. If the blade pitch forinstance is varied with angle θ, the probing direction may equally bevaried with a corresponding angle in order to compensate for the pitchchange.

It is recognised that the probing angle may be varied in various ways.As shown in FIG. 13 it is for instance possible to vary the position (orangle) of incoming light on a transmitting lens located at the emissionpoint of the optical measurement system. This may for instance beobtained by moving a light source, e.g. a laser diode or thetransmitting end of an optical fibre, in a substantially transversedirection relative to the incoming light (or the transmitting lens).Alternatively or in addition thereto, it is possible to vary theposition of the transmitting lens relative to the incoming light asshown in FIG. 14, e.g. by moving the transmitting lens in asubstantially transverse direction relative to the incoming light.

The invention has been described with reference to a preferredembodiment. However, the scope of the invention is not limited to theillustrated embodiments, and alterations and modifications can becarried out without deviating from the scope of the invention, which isdefined by the claims.

LIST OF REFERENCE NUMERALS

2 wind turbine

4 tower

6 nacelle

8 hub

10, 110, 210, 1010, 1010′, blade 1210

14 blade tip

16 blade root

18 leading edge

20 trailing edge

30 root region

32 transition region

34 airfoil region

50 airfoil profile

52 pressure side

54 suction side

56 leading edge

58 trailing edge

60, 260 chord

62 camber line/median line

64 direction of rotation

66 lift

68 drag

70 resultant aerodynamic force

72 axial force (thrust)

74 tangential force

80, 180, 280, 280′, 380, laser anemometer 480, 580, 680, 780, 880, 980,980′, 1080, 1280

81, 581 laser anemometer

82, 582 laser anemometer

90, 590 flow altering means/flap

91, 591 flow altering means/flap

92, 592 flow altering means/flap

185, 285, 285′ emission point

186, 286, 286′ probing beam

187, 287, 287′, 1287 probing region/probing volume

288 normal

315, 415, 1015 central axis

325, 425, 1025, 1125 circle

585 laser source

586 splitter/multiplexer unit

587, 588, 589 light guides/optical fibres

674, 774 light source

675, 775, 875 beam splitter

676, 776, 876 reference mirror

677, 777 lens system

678, 778 window

679, 779, 879 photo detector

693, 793 amplifier

694, 794 signal processor

695, 795 electrical output

896 beam splitter/Wollaston prism

1127 upwind plane

C chord length

d_(t) position of maximum thickness

d_(f) position of maximum camber

F camber

f_(L) probing length

r·ω rotational velocity

T thickness

v_(a) axial velocity

v_(r) resultant velocity/inflow velocity

v_(w) wind speed

α angle of attack

δ probing angle

θ pitch angle

φ inflow angle

The invention claimed is:
 1. A wind turbine comprising a number ofblades including at least a first wind turbine blade extendingsubstantially radially from a hub on a main shaft having a substantiallyhorizontal centre axis, the blades together with the hub constituting arotor with a rotor plane, and which can be put into rotation by wind,and each blade having an innermost part having a root section of theblade and an outermost part having a tip section of the blade, whereinthe wind turbine has an optical measurement system comprising a laser asa light source, an optical transmitter part, an optical receiver part,and a signal processor, wherein the light source is optically coupled tothe optical transmitter part, the optical transmitter part comprises anemission point and is adapted for emitting light in a probing directionfrom said emission point, the optical receiver part comprises areceiving point and a detector, wherein the optical receiver part isadapted for receiving a reflected part of light from a probing regionalong the probing direction at the receiving point and directing saidreflected part of light to the detector so as to generate a signal fromthe detector based on the received, reflected light, and the signalprocessor is adapted to determine at least a first velocity component ofthe inflow from the signal generated by the optical receiver part,wherein the emission point of the optical transmitter part is located inthe first blade at a first radial distance from the centre axis, and thereceiving point of the optical transmitter part is located in the firstblade at a second radial distance from the centre axis, wherein theoptical measurement system is adapted to emit at least a first probingbeam and a second probing beam, the first and second probing beams beingarranged to emit light in separate divergent directions to probe windvelocities in at least two different probing volumes.
 2. A wind turbineaccording to claim 1, wherein the first radial position is substantiallyidentical to the second radial position.
 3. A wind turbine according toclaim 1, wherein the first blade further comprises adjustable flowaltering means, such as distributed actuators, flaps or microtabs, foradjusting an aerodynamic parameter of the blade and located in a thirdradial distance from the hub, the adjustable flow altering means beingcontrolled by a controlling means, and wherein the controlling means areadapted to receive a signal from the signal processor, the signal beingbased on at least the first velocity component, optionally with thethird radial position being substantially identical to the first radialposition.
 4. A wind turbine according to claim 1, wherein the windturbine comprises a second wind turbine blade, and wherein the secondwind turbine blade is provided with adjustable flow altering means, suchas distributed actuators, flaps or micro tabs, for adjusting anaerodynamic parameter of the second blade, the adjustable flow alteringmeans being controlled by a controlling means, and wherein thecontrolling means are adapted to receive a signal from the signalprocessor, the signal being based on at least the first velocitycomponent.
 5. A wind turbine according to claim 1, wherein the opticalmeasurement system is adapted for probing the velocity component in arange of 0.5-1 Om, or 0.75-Bm, or 1-5 m from the emission point.
 6. Awind turbine according to claim 1, wherein the emission point and/or thereceiving point of the first wind turbine blade is located between aleading edge of the first blade and a point of maximum thickness on apressure side of the blade, preferably with the probing direction lyingin a quadrant between a chord direction, seen from the leading edge ofthe blade, and a normal perpendicular to said chord direction andextending from the pressure side of the blade.
 7. A wind turbineaccording to claim 1, wherein the emission point during rotation of therotor follows a concentric circle having a radius corresponding to thefirst radial distance from the centre axis, and wherein the probingdirection is substantially arranged tangentially to said concentriccircle, alternatively with the optical system being adapted to probewind speeds in a probing volume located substantially at the firstradial distance from the centre axis.
 8. A wind turbine according toclaim 1, wherein the optical measurement system is adapted for probingwind speeds in a probing volume located in an upwind plane upwind of therotor plane.
 9. A wind turbine according to claim 1, wherein the lightsource is separated from the emission point, the light source beingoptically connected to the emission point by a light guiding means. 10.A wind turbine according to claim 1, wherein at least the transmitterpart, the receiver part and the detector are arranged in a single, firstunit in the first wind turbine blade.
 11. A wind turbine according toclaim 10, wherein the first unit is arranged in a bushing, wherein thebushing is a sleeve tube, in the first wind turbine blade.
 12. A windturbine according to claim 1, wherein the first probing beam and thesecond probing beams form a probing angle lying in an interval of 5-90degrees, or advantageously 7-75 degrees, or advantageously 10-60degrees.
 13. A wind turbine according to claim 1, wherein the firstprobing beam and the second probing beam are oriented substantially in across- sectional plane of a local cross-section of the blade.
 14. A windturbine according to claim 1, wherein the first wind turbine blade has ablade length (L), and wherein the emission point and the receiving pointare located within a blade length interval of 0.2L to 0.9L.
 15. A windturbine according to claim 1, wherein the first wind turbine blade has ablade length (L), and wherein the probing region is located at aposition in which the wind impacts the first wind turbine blade or asecond wind turbine blade within a blade length interval of 0.5L to0.9L.
 16. A wind turbine according to claim 1, wherein the first windturbine blade is pitchable, and wherein the optical measurement systemcomprises compensation means for compensating for a pitch angle of thefirst blade.
 17. A wind turbine according to claim 1, wherein the firstwind turbine blade is pitchable, and wherein the probing direction isvariable in dependence on a pitch angle of the first blade.
 18. A windturbine according to claim 1, wherein the probing direction is variablein dependence on a rotational speed of the rotor.
 19. A wind turbineaccording to claim 17, wherein the optical measurement system comprisesa unit comprising the transmission part which is variable in angle inrelation to the first wind turbine blade.
 20. A wind turbine accordingto claim 17, wherein the optical measurement system is adapted to vary aposition of incoming light on a transmitting lens.
 21. A wind turbineaccording to claim 17, wherein the optical system is adapted to vary aposition of a transmitting lens.
 22. A method of operating a windturbine comprising a number of blades including at least a first windturbine blade extending substantially radially from a hub on a mainshaft having a substantially horizontal centre axis, the blades togetherwith the hub constituting a rotor with a rotor plane, and which can beput into rotation by wind, and each blade having an innermost parthaving a root section of the blade and an outermost part having a tipsection of the blade, wherein the method comprises the steps of a)emitting light in a probing direction from an emission point on thefirst wind turbine blade, said emission point being located in a firstradial distance from the centre axis, b) receiving a reflected part oflight from a probing region along the probing direction at a receivingpoint located on the first wind turbine blade at a location in a secondradial distance from the centre axis, c) directing said reflected partof light to a detector, d) generating a signal based on detected lightin step c), and e) calculating a first velocity component based on thesignal from step d) wherein light is emitted from at least a firstprobing beam and a second probing beam, the first and second probingbeam being arranged to emit light in separate divergent directions toprobe wind velocities in at least two different probing volumes.
 23. Amethod according to claim 22, wherein the method further comprises thestep of: f) adjusting adjustable flow altering means on the first windturbine blade in order to adjust an aerodynamic parameter of the firstwind turbine blade, the adjustable flow altering means being located ata third radial distance from the centre axis.
 24. A method according toclaim 22, wherein the method further comprises the step of adjusting theprobing direction in dependence on a pitch angle of the first windturbine blade and/or a rotational velocity of the rotor.